In typical PEM fuel cell systems, a fuel such as methane or a similar hydrocarbon, is used as a source of the hydrogen for the anode side of the fuel cell. In many systems, particularly those of the stationary power generation type, humidified natural gas and air are chemically converted to a hydrogen-rich gas stream known as reformate by a fuel processing subsystem of the fuel cell system. During this reforming process, the level of carbon monoxide (CO) must be reduced to a low level (typically less than 10 ppm) since the PEM fuel cell membrane is easily poisoned by CO.
The reforming reaction is typically an oxygenolysis reaction with an associated water-gas shift [CH4+H2O→CO+3H2, CO+H2O→CO2+H2] and/or partial oxidation reaction [CH4+0.5O2→CO+2H2]. While the water-gas shift reaction associated with steam reforming removes some of the CO from the reformate flow stream, the overall product reformate gas will always contain some level of CO, the amount being dependent upon the temperature at which the reforming process occurs. In this regard, the CO concentration of the reformate flow is normally well-above the acceptable level for the PEM fuel cell membrane. To reduce the CO content within acceptable levels, several catalytic reactions will typically be used in the fuel processing subsystem to cleanup, i.e. reduce, the CO in the reformate flow. These catalytic reactions require that the reformate flow temperature be within relative precise temperature ranges. Typical reactions for reducing CO include the aforementioned water-gas shift, as well as selective oxidation of the CO over a precious metal catalyst in a selective or preferential oxidizer (PrOx), typically with a small amount of air added to the reformate flow to provide oxygen for the catalytic reaction. Often, several stages of CO cleanup are required before the CO content is sufficiently reduced, with each stage typically requiring that the reformate temperature be reduced to a precise temperature range so that the desired catalytic reaction will occur. In this regard, liquid-cooled heat exchangers are frequently employed to control the reformate temperature at each stage.
However, the use of liquid-cooled heat exchangers for the above purpose presents a challenge in successfully maintaining the required temperatures during turndown (reduced power) operation of the fuel cell system wherein the flow rate of the reformate flow is reduced from that required for full power operating conditions. Specifically, because the heat exchangers need to be designed for the appropriate heat transfer effectiveness at full flow rate, the heat exchangers will typically be too effective when the flow rate of the reformate flow is reduced. This can be mitigated by adjustment of the coolant flow rate (via control of the coolant pump or by-pass valving) and/or the coolant temperature (via recirculation of a portion of the coolant flow). However, this is complicated by the distribution of the CO cleanup into several different reactions, each of which requires a heat exchanger to control the temperature of the reformate flow. Because the heat exchangers will typically not all have the same response to the above described adjustments at turndown, the coolant flow and/or temperature to each heat exchanger may have to be separately controlled, resulting in a relatively complicated coolant control scheme with duplicate by-pass valves, recirculation pumps, etc.